The conventional dietetic approach to feedlotting of ruminants is to feed high proportions of grain (&gt;70%) and low proportions of roughage (&lt;20%) in the diet. The aim of this strategy is to maximise the digestibility of the grain and its constituent nutrients including starch and proteins in the rumen. However, there is considerable variation in the starch content of different grains. For example, wheat contains about 70-75% starch, corn and sorghum 68-72% starch, and oats and barley 56-60% starch. Further, there are large differences in the rumen digestibility of starch for the different grains. For example, the starch sources present in wheat and barley are highly digested in the rumen (84-90%) whereas that of sorghum is much lower (60-65%). Such variation in starch content and rumen digestibility results in major differences in feedlot performance and in turn this has significant implications for profitability.
When assessing the nutrient requirements of animals in feedlots it is also necessary to take into account their stage of physiological growth and maturity. For example, at the beginning of beef feedlotting cattle invariably undergo an adaptation phase which involves transition from free range grazing to intensive feedlotting, and commensurately, there is a major change in nutrient supply from a forage to a grain based diet. In effect, this creates an enormous metabolic challenge for the animal and invariably results in compensatory phases of growth occurring. This generally occurs in the first 40-50 days of feedlotting and during this period there is an additional demand for essential amino acids, essential fatty acids and glucose to satisfy intestinal growth and development, which increases during this compensatory/adaptation phase. A further example where different supplies of nutrient are required is in fat deposition and in particular the accumulation of intramuscular fat--a process commonly referred to as marbling.
Smith & Crouse.sup.1 have demonstrated that glucose is the major precursor for fat synthesis in muscle, and this is in contrast to subcutaneous fat deposition where acetate provides the majority of carbon units for long chain fatty acid synthesis. It is also important to note that glucose is the preferred fuel for use by the visceral region of the ruminant species. Therefore, the dietetic challenge is to maximise the end products of rumen fermentation from high grain diets and in addition include protected or bypass feed supplements that increase the supply of specific nutrients, including essential amino acids, essential fatty acids and glucose, to the small intestine for utilisation and absorption.
Australian Patent No. 659 557 discloses the feeding of a supplement containing both protected proteins and protected fats which can be fed in varying ratios to improve growth rate, feed conversion and carcass quality. According to one example the feeding of higher proportions of fat to Hereford cross steers of average weight 470 kg for 175 days increased the fat content by about 10%. Other studies McCartor & Smith.sup.2, McCartor et al..sup.3, Haaland et al..sup.4 and Dinius et al..sup.5 have shown that feeding protected fats without protected proteins will also increase fat content. Similar increases were observed by Zinn.sup.6,7 and Brandt8, when fat supplements not protected from ruminal metabolism were included in the diet.
The increase in fact content occurs mainly in subcutaneous fat, kidney, pelvic, heart and intestinal fat. Variable responses have been found in marbling or intra-muscular fat deposition. For example, McCartor & Smith.sup.2 reported a small increase (P&lt;0.10) in marbling score in Brahman and Hereford steers but in a further study (McCartor et al..sup.3) the same group could not demonstrate any significant effect on marbling in 200 steers of mixed breed type. Zinn.sup.6,7 and Brandt.sup.8 also had inconsistent results in improving marbling performance with fat supplementation. Other reports, such as Haaland et al..sup.4 and Dinius et al..sup.5 found no differences in carcass quality.
There are also similar variations in marbling and fat deposition when animals are fed conventional grain sources (eg. corn, barley, sorghum, wheat), which are processed prior to feeding. For example, preparing dry rolled corn processed to produce a finer particle size in distribution that is, a geometric mean diameter (G.M.D) 0.76 mm, compared to commercially processed rolled corn (G.M.D &gt;2.6 mm), improved feed conversion and marbling. Furthermore, steam flaked wheat improved daily gain, feed conversion and increased subcutaneous fat deposition, but had no effect on marbling (Owens et al..sup.9 ; Zinn.sup.10).
The digestion of starch in the small intestine improves energy efficiency by 30-40% (Owens et al..sup.11), and also would increase the availability of glucose, either directly or indirectly, for fat synthesis within the muscle. The "trigger" for increased starch digestibility in the small intestine is an increase in activity of the enzymes, that is, amylases and maltases that hydrolyse the glycosidic linkages to produce glucose. Insulin is a key hormone in increasing pancreatic amylase activity, wherein pancreatic amylase is an enzyme critical for starch digestion. Insulin also regulates glucose transport into cells and enhances fat synthesis. Insulin levels can be increased by enhancing the amount of protein digested in the small intestine, and this is achieved by feeding protected proteins. Insulin sensitivity, that is, the binding of this hormone to its receptor on the cell membrane can also be increased by enhancing the C.sub.18 unsaturated fatty acid composition of the membrane phospholipids. An increase in sensitivity will augment glucose transport and utilisation with the cell. Such metabolic changes are achieved by feeding protected lipids.
The present invention discloses that protected nutrient supplements can be produced to enhance the supply and absorption of essential amino acids, essential fatty acids and glucose to and from the small intestine and these alone, or in combination with each other, increase growth and feed conversion efficiency during the starter/adaptation phase and substantially improve marbling scores of beef cattle. Improved marbling has significant economic implications because of the premium price paid for marbled beef.